Recently, photonic crystals have been drawing attentions as a new optical device. A photonic crystal is a functional material having a periodic distribution of refractive index, which provides a band structure with respect to the energy of light or electromagnetic waves. One of its particular features is that it has an energy region (called the photonic bandgap) that forbids the propagation of light or electromagnetic waves.
Introduction of an appropriate defect into the distribution of refractive index in the photonic crystal will create an energy level (called the defect level) due to the defect within the photonic bandgap. This allows only a specific wavelength of light having an energy corresponding to the defect level to exist within the wavelength range corresponding to the energy levels included in the photonic bandgap. Forming a linear defect in the crystal provides a waveguide, and forming a point-like defect in the crystal provides a resonator.
Photonic crystals can be classified into two-dimensional and three-dimensional crystals. Each of them has its own features and advantages. For example, two-dimensional crystals are advantageous in that they are relatively easy to manufacture. The Japanese Unexamined Patent Publication No. 2001-272555 discloses a two-dimensional photonic crystal with cylindrical holes periodically arranged in a triangular lattice pattern to provide a periodic distribution of refractive index, in which a linear zone devoid of the cylindrical holes is formed as a waveguide ([0025], FIG. 1) and a point defect is formed in proximity to the waveguide ([0029], FIG. 1). The Japanese Unexamined Patent Publication No. 2001-272555 includes the analysis of an embodiment of a point defect that is formed by increasing the diameter of the periodically arranged cylindrical holes.
In the Japanese Unexamined Patent Publication No. 2003-279764, the applicant of the present invention has proposed the creation of a cluster defect by making defects of two or more pieces of modified refractive index areas adjacent to each other within a plurality of modified refractive index areas forming a periodic distribution of refractive index. The defects of the modified refractive index areas are formed by making the refractive index of the desired part of the modified refractive index areas different from that of the rest of the modified refractive index areas. A defect having a refractive index lower than that of the rest of the modified refractive index areas is called the acceptor type defect, whereas a defect having a higher refractive index is called the donor type defect. The defect disclosed in the Japanese Unexamined Patent Publication No. 2001-272555, which is created by enlarging the cylindrical hole, is an acceptor type defect, whereas a defect created by not providing the modified refractive index area is a donor type defect. The cluster defect and a point defect created by the absence of a single piece of modified refractive index area are generally called the “point-like defect.”
The two-dimensional photonic crystal with a point-like defect or point like defects can be used for various purposes, a typical example of which is the optical multiplex communication. Recently, optical multiplex communications use the wavelength division multiplexing in which plural wavelengths of light, each carrying a different signal, propagate through a single transmission line. With a waveguide accompanied by a plurality of point-like defects each having a defect level corresponding to each wavelength, the two-dimensional photonic crystal can be used as a demultiplexer for taking out a specific wavelength of light (i.e. signal) through each point-like defect from the light propagating through the waveguide, or as a multiplexer for introducing a specific wavelength of light through each point-like defect into the waveguide.
In the case of taking out a ray of light from a point-like defect (i.e. optical resonator) of a demultiplexer or a similar device, if the defect has a cylindrical shape or a similar shape symmetrical in the direction orthogonal to the surface of the two-dimensional photonic crystal, the light is emitted from both sides with the same intensity. However, if the two-dimensional photonic crystal is mounted on a substrate, the light emitted from the side facing the substrate is lost, and only the light emitted from the free side of the crystal is available. Taking this into account, in the Japanese Unexamined Patent Publication Nos. 2001-272555 and 2003-279764, research is conducted on methods for controlling the ratio of the emission intensity of the light from one side to that of the light from the other side (this ratio is called the “front/back emission ratio” hereinafter) by making the point-defect asymmetrical between the two sides. This increases the emission intensity from one side and accordingly improves the emission efficiency (or take-out efficiency) of light. For example, the Japanese Unexamined Patent Publication No. 2001-272555 discloses a cone-shaped point defect functioning as an acceptor type defect ([0032], FIG. 5) and a point defect having different diameters on both sides ([0032], FIG. 6).
However, it is not easy to create a point-like defect by asymmetrically working the body of the two-dimensional photonic crystal itself on both sides. For example, the Japanese Unexamined Patent Publication No. 2001-272555, in [0039] and [0040], describes a method of creating a point-like defect by masking one face of a crystal with a photoresist, except for the area corresponding to the defect, and etching the face with a reactive ion beam. This method is capable of forming a point-like defect having a cylindrical shape or a similar shape that is symmetrical between the two sides, but not capable of forming a point-like defect having a conic shape or a similar shape that is asymmetrical between the two sides.
In view of the aforementioned problem, the present invention aims to provide a two-dimensional photonic crystal having a point-like defect or point-like defects that enable the control of the front/back emission ratio of light and are easier to manufacture than conventional defects.